A magnetic recording medium comprising a non-magnetic support having provided thereon a magnetic layer containing ferromagnetic particles, such as ferromagnetic iron oxide or ferromagnetic alloys, dispersed in a binder has conventionally been used as an audio tape, a video tape, a computer tape, a magnetic disk and the like.
With the recent development of technologies in this field, higher density recording and higher efficiencies have been required for a magnetic recording medium. Polyurethane type binders, those binders mixed with other resins, and those binders mixed with a low molecular weight agent which provides lubricating properties are used, particularly to improve wear resistance.
However, when such binders are used, the film strength of a magnetic layer is insufficient, resulting in powder dropping (powder production) which causes dropouts. Heat resistance is also in sufficient. Also, when the binders are used in combination with other resins, their compatibility is not good, and when the binders are used in combination with a low molecular weight compound, blooming occurs, resulting in head clogging of a magnetic head.
To overcome the above problems, it has been proposed to use as a binder polycarbonate polyester polyurethane (disclosed in JP-A-58-60430), and polycarbonate polyurethane (JP-A-59-198530 and 60-13324). (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). It is also known to use phenoxy resins as a durable hard binder, as disclosed in U.S. Pat. No. 4,448,846, and to use fatty acid modified silicone as a lubricating agent to improve wear resistance (as disclosed in JP-B-56-26890). (The term "JP-B" as used herein means an "examined Japanese patent publication").
However, when polyester polyurethane is used as a binder, there is the problem that film strength is decreased. When polycarbonate polyurethane is used as a binder, there is also the problem that durability at a high temperature and a high humidity is relatively deteriorated, even though film strength and durability are improved.
It is proposed in JP-A-61-9830 that polycarbonate polyurethane resins and isocyanate hardening agents be used as a binder to further improve film strength and durability. However, this approach is not satisfactory to improve still durability and to prevent powder dropping.
A phenoxy resin is a hard binder having good durability, but is fragile and brittle, and thus powder is readily produced. Due to its brittleness, durability after repeated running and still durability is insufficient.
It is known to use fatty acid modified silicon in combination with a conventional polycarbonate polyurethane to decrease the dynamic friction coefficient and to improve durability and blooming, but the durability of such media at a high temperature and a high humidity is not sufficient.
Durability can be improved to some extent by using an abrasive agent in combination with a binder, but durability under a high temperature and a high humidity requires further improvement.